This invention pertains to the art of blood pumps and more particularly to rotary blood pumps. The invention is applicable to a seal arrangement for rotary blood pumps and will be described with particular reference thereto. However, it will be appreciated that the invention has broader applications and may be advantageously employed in other rotary mechanisms particularly adapted for biocompatible and medicinal purposes.
Rotary blood pumps are well known in the art as exhibited by the following U.S. Pat. Nos.:
Reich, et al. - U.S. Pat. No. 4,135,253 PA0 Wampler - U.S. Pat. No. 4,625,712 PA0 Moise - U.S. Pat. No. 4,704,121.
Each of the structures described in the patents has various advantages and drawbacks over other rotary blood pumps. Nevertheless, all are deemed to be particularly adapted to cardiac assist.
A primary concern in the rotary blood pump technology is the need for an effective seal between a rotary member and its associated stationary housing. In Reich, et al. a polymer lip on a baffle plate cooperates with an impeller shaft to limit blood flow from a pump chamber into a rotor chamber. A saline solution is preferably circulated throughout the rotor chamber for lubrication purposes and thus comes in contact with the seal along the impeller drive shaft. The saline solution is maintained under pressure and can prevent blood in the pump chamber from entering the rotor chamber. The pressurized saline produces a reverse flow of saline solution from the rotor chamber to the pump chamber. Per Reich, et al., a purge flow on the order of 20 ml/day can be expected. This saline flow produces a small separation between the stationary and moving seal components, as a result of the pressure and elasticity of the polymer lip on the baffle plate. Since saline solution is biocompatible, such a reverse flow has no adverse effect on the patient.
The Moise and Wampler patents are commonly assigned and illustrate two types of rotary blood pumps; namely, a radial flow pump in Moise and an axial flow pump in Wampler. Both of these patents also disclose the use of a purge seal utilizing a biocompatible fluid between rotary and stationary members similar in function to the seal and saline arrangement in Reich, et al.
In Wampler, the areas of potential contact are between the face seal and the stator hub, and the bearing flange and the hub. A balance between the external forces, the pressure forces, and hydrodynamic action between the contacting faces finally determines the relative clearances. In Moise, the stationary and moving seal surfaces are to be so manufactured and supported that there is a fixed, uniform clearance of micron-sized order of magnitude between these components. In all three patents discussed above, a fluid such as saline or a blood filtrate is intended to flow through the seal gap providing cooling, a flushing action to prevent cell build-up, and lubrication of the near-contact.
Although in theory and limited application such a purge seal arrangement may be effective, it also has drawbacks. A primary disadvantage to the prior art arrangements is the use of a contact-type seal for sealing along the drive shaft. Contact-type seals have the obvious disadvantage of wearing over time. Such wearing is highly critical when the fluid being handled is blood and the consequences of contamination, hemolysis, thrombosis, and the like could result. Heat is also generated at the contact points. Additionally, the disclosed contact-type seals (Reich, et al. and Wampler) can have a variable gap and leakage flow due to the many factors involved in their equilibration at working conditions.
The Moise seal arrangements already known in the art are unworkable from a practical manufacturing standpoint. The requirement for close and uniform clearances between the housing and rotary members so as to limit purge flow into the blood stream becomes unreasonable. It simply is too expensive a procedure to maintain the close tolerances and still provide a cost effective rotary blood pump that effectively seals.